While fewer than 2 percent of the American population is underweight, for those who try without success to put on pounds, the experience can be frustrating and, occasionally, of medical concern. Being underweight can be the result of an underlying condition: a hyperactive thyroid, a condition that causes "malabsorption" such as Celiac disease, or a bowel problem such as crohn's disease. So the first step in a weight-gain journey should be a trip to the doctor's office.

If deemed otherwise healthy-and with your doctor's blessing-a number of strategies can help improve your odds of gaining weight in a safe, effective, and long-term way. Just as with weight loss, there is definitely a healthful and an unhealthful way to do it. In many ways, bulking up is an exercise in restraint just as much as slimming down can be.

"If you're trying to gain weight healthfully, don't fall into the trap of chowing down on pints of ice cream, cheeseburgers, and donuts," Cynthia Sass, M.P.H. R.D. tells HuffPost Healthy Living. "The old phrase, 'You are what you eat' is absolutely true-nutrients from food are literally the raw materials your body uses to construct new cells."

Her number one tip? Eat frequently. "Don't let over three to four hours go by without eating," she says. "Your body is like an engine that's always turned on, so it needs a continuous supply of energy. When you skip meals or go long stretches without eating, you deprive it of the fuel it needs to keep going. The result is a dip into its energy piggy bank, which unfortunately includes muscle mass."

Start small-just an extra 200 calories a day, recommends trainer Jen Cassetty, to see how well your body responds. It's easy to overdo it, so it's best to start with a modest change.

The Mayo Clinic's Katherine Zeratsky, R.D., L.D. suggests a few strategies that read like the diametric opposite of weight loss advice, such as add small, healthful but nutrient dense snacks between meals: "Snack on nuts, peanut butter, cheese, dried fruits, and avocados," she says.

Drinking calories is another great way to add calories, but that doesn't mean you should head for sugar-sweetened beverages, which can create health problems associated with metabolic disorder, including high blood sugar levels and other risk factors for diabetes and heart disease. Try something healthful and packed with nutrients, like a smoothie of almond milk, berries, bananas, and flax seeds.

If extra calories won't do the trick alone, consider adding some strength training to bulk up with muscle mass. That will require even more calories, according to medical nutritionist (and HuffPost blogger) Dr. Melina Jamplolis, writing for CNN. "Your body can build at most about a half-pound of muscle each week, so if you eat too many extra calories trying to build more muscle, you will gain fat, too. I would suggest consuming an extra 250 to 500 calories per day," she says.

Whatever you do, make sure that proper nutrition and slow gain are central parts of your plan.